1. Field of the Invention
The present invention relates in general to analog modulators and in particular, to circuits and methods for gain ranging in an analog modulator and systems using the same.
2. Description of the Related Art
Optical disks have been used for many years for the mass storage of digital data. Some well known examples of optical disks include digital audio compact disks (CD-DAs), compact disk read-only memories (CD-ROMs) and digital video or digital versatile disks (DVD-ROMs). Essentially, digital data is stored on a plastic disk as a series of pits in the disk surface. During playback, a beam of light is directed to the rotating disk surface and the intensity of the photons reflected from the pits measured. A modulated electrical signal is generated that can be processed and the data stored on the disk recovered.
A basic configuration for the read (playback) mechanism has developed over a number of years. This configuration includes a pickup or sled which is movable so that a laser, a lens, and array of photodiodes can be positioned directly over the data being read off of the disk. As the disk turns, the photons from the laser are reflected off the pits and lands received by the photodiodes which generate electrical signals having a current that is proportional to photon density.
The multiple signals output from the photodiodes represent both data detection and servo alignment information. The summation of the high speed data channel signal, which may be composed of the signals A+B+C+D from an astigmatic photodiode array, results in a composite signal with relevant information between approximately 10 KHz and 60 MHz for current DVD players. Servo information contained in these signals however, is at frequencies less than 1 MHz (for current spindle rotation rates of &lt;6000 RPM). Because of these information rates, the data channel signal is sometimes AC-coupled to the data detection and summation circuitry mounted on an accompanying stationary circuit board. Otherwise, some degradation of the dynamic range must be accepted due to the dc content of the incoming signal.
The typical current signal generated by a photodiode is on the order of 1 .mu.A. Transferring this signal directly down a flexible cable to the stationary circuit board therefore would seriously degrade the signal to noise ratio. Hence, transimpedance amplifiers, which convert the current from the photodiode array into a voltage for driving the cable, are mounted in the pickup to boost the signal and minimize noise and interference effects. The data detection, error correction, and servo systems are kept off of the pickup not only because of the same interference reasons, but also to reduce the physical size and mass of the sled. These systems are mostly digital, and switching noise on the pickup may degrade the signal to noise ratio of the diode signal.
Depending on the manner in which the photodiodes and transimpediance amplifiers are configured, the dc offset of the signal passed to the flex cable can differ with respect to the system reference voltage. As a result, the front-end processing integrated circuits (IC) must typically be customized to operate with a specific type of pickup configuration. Flexibility is lost, as different chip versions must be designed and produced as a function of the corresponding pickup.
Thus, a need has arisen for circuits and methods which allow an integrated circuit operating in an optical disk system to be compatible with multiple pickup configurations.